1. Field of the Invention
The present invention relates generally to composites having a titanium aluminide matrix, reinforced by, for example, silicon carbide fibers or filaments. More particularly, it relates to improvements inhibiting formation of microcracks in the titanium aluminide matrix.
2. Background of the Prior Art
In recent years there has been a dramatic increase in the performance requirements demanded from aerospace structural materials. Inasmuch as composite materials made from a titanium base alloy, reinforced with high strength/high stiffness filaments or fibers, have been shown to exhibit very high strength properties in relation to their weight and have the potential to extend the maximum use temperature of titanium well into the range of 650.degree. to 750.degree. C. (which temperatures exist in many advanced air frame and turbine engine environments), such composites have been the subject of intense study as a most promising class of materials.
However, with few exceptions, the tensile strength of the earlier titanium composite materials has not measured up to the values theoretically possible vis-a-vis the rule-of-mixtures (ROM) values. Furthermore, the fatigue properties of the material are poor.
Titanium aluminides, based on their ordered Alpha 2 Ti.sub.3 Al phase, are currently considered to be one of the most promising group of titanium alloys for fiber-reinforced composites. However, the Ti.sub.3 Al ordered phase is very brittle at lower temperatures and has low resistance to cracking under cyclic thermal conditions. Consequently, groups of alloys based on the Ti.sub.3 Al phase, modified with Beta stabilizing elements, such as niobium, molybdenum, and vanadium have been contemplated to improve room temperature ductility and to improve resistance to cracking. However, these benefits are accompanied by decreases in high temperature properties. Although the Beta stabilizer niobium, at a maximum of about 11 atomic percent (21% by weight) has been preferred over other Beta stabilizers for optimum balance of low and high temperature properties in unreinforced matrices, even niobium is problematic in fiber-reinforced composites.
Where titanium aluminide matrices (even modified with niobium) are reinforced with, for example, silicon carbide fibers, Beta phase depletion zones have been observed at the fiber/matrix interface. The Beta depletion zone surrounds the fiber as an essentially pure Alpha 2 region having inherently low resistance to thermal cracking. Thus, during high temperature bonding between the fiber and the matrix, microcracks which initiate in the reaction zone, can propagate through the Beta depletion zone and on through the composite.
Until now, the Beta depletion zone has been attributed to interfacial reaction products serving to deplete the volume of niobium at the vicinity of the fiber/matrix interface. Accordingly, additional sacrificial amounts of niobium, either in powder or plasma form, have been sprayed onto the surface of the silicon carbide fibers to replenish the depleted volume of niobium.
For example, U.S. Pat. No. 4,978,585 discloses a method for vaporizing aluminum from the alloy powder prior to consolidating the alloy powder and the fibers. This method therefore enhances the relative proportion of niobium in the matrix and enhances the volume of Beta crystals.
U.S. Pat. Nos. 5,017,438 and 5,045,407 disclose methods for plasma spraying pure niobium or niobium alloy powder onto the fibers, followed by consolidation of the niobium plasma sprayed fibers with additional niobium doped titanium alloy matrix powder. A one micron thick diffusion zone of, for example, pure niobium surrounds each fiber, into which there is some titanium alloy interdiffusion.
U.S. Pat. No. 5,030,277 discloses coating silicon carbide fibers with a sacrificial excess of, preferably, niobium, mixed with polystyrene binder. Then consolidation of the coated fibers with normal alloy powder proceeds. This process also enhances the volume fraction of Beta stabilizing material at the fiber/matrix interface.
These prior art processes have several drawbacks. First of all, where the fibers are coated with niobium-doped titanium alloy powders having excess niobium, the volume fraction of Beta phase may increase, but the thermodynamic stability at the fiber/matrix interface continues to favor formation of Alpha 2 phase materials. Since the stability does not shift towards the Beta phase, there remains an envelope of continuous Alpha 2 phase material, i.e., a Beta depletion zone. Although the Beta depletion zone is of less volume than exists without coating the fiber with the sacrificial niobium alloy, there is, all the same, a continuous Alpha 2 phase at the interface.
Secondly, where pure niobium metal powder, or pure mixtures of Beta solute powders are coated onto the fiber, even though there is some diffusion of titanium matrix material, the composite, at its interface, is constituted predominantly of the more dense transition metal, and niobium is particularly dense. Accordingly, there exists a different intermetallic structure at the interface, devoid of titanium intermetallic Beta constituents. This more dense material, although lacking pure Alpha 2, is all the same unsuitable for lightweight composite structure design where the less dense titanium intermetallic characteristics are desired.
Furthermore, the multiplicity of steps, such as conducting controlled evaporation, coating of the fibers, etc, is more cumbersome and less efficient than if no such steps were required.
Therefore, it is a principal object of the present invention to provide novel silicon carbide reinforced titanium composites.
It is a further object to provide such composites with the ordered alloys of Ti.sub.3 Al+niobium, but having stability shifted towards the Beta phase at the fiber/matrix interface.
It is an additional object of the present invention to provide novel fiber-reinforced titanium composite materials without a Beta depletion zone, but less dense and better suited for elevated temperature applications where lighter material is essential, as in, for example, turbine engines.
It is a further object of the present invention to negate the need for coating silicon carbide fibers with pure metal or alloy powders prior to consolidation with the principal alloy of the matrix.
It is a further principal object of the present invention to provide an improved, crack-free titanium aluminide, fiber-reinforced composite material.